Phase Lock Loop Circuit Based Adjustment of a Measurement Time Window in an Optical Measurement System

ABSTRACT

An illustrative system may include a TDC configured to monitor for an occurrence of a photodetector output pulse during a measurement time window that is within and shorter in duration than a light pulse time period, the photodetector output pulse generated by a photodetector when the photodetector detects a photon from a light pulse having a light pulse time period; a PLL circuit for the TDC and having a PLL feedback period defined by a reference clock, the PLL circuit configured to: output a plurality of fine phase signals and output one or more signals representative of a plurality of feedback divider states during the PLL feedback period; and a precision timing circuit configured to adjust, based on one or more of the fine phase signals and/or the feedback divider states, a temporal position of the measurement time window within the light pulse time period.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/202,572, filed on Mar. 16, 2021, which claims priority under35 U.S.C. § 119(e) to U.S. Provisional Patent Application No.62/992,497, filed on Mar. 20, 2020, and to U.S. Provisional PatentApplication No. 63/027,018, filed on May 19, 2020. These applicationsare incorporated herein by reference in their respective entireties.

BACKGROUND INFORMATION

Detecting neural activity in the brain (or any other turbid medium) isuseful for medical diagnostics, imaging, neuroengineering,brain-computer interfacing, and a variety of other diagnostic andconsumer-related applications. For example, it may be desirable todetect neural activity in the brain of a user to determine if aparticular region of the brain has been impacted by reduced bloodirrigation, a hemorrhage, or any other type of damage. As anotherexample, it may be desirable to detect neural activity in the brain of auser and computationally decode the detected neural activity intocommands that can be used to control various types of consumerelectronics (e.g., by controlling a cursor on a computer screen,changing channels on a television, turning lights on, etc.).

Neural activity and other attributes of the brain may be determined orinferred by measuring responses of tissue within the brain to lightpulses. One technique to measure such responses is time-correlatedsingle-photon counting (TCSPC). Time-correlated single-photon countingdetects single photons and measures a time of arrival of the photonswith respect to a reference signal (e.g., a light source). By repeatingthe light pulses, TCSPC may accumulate a sufficient number of photonevents to statistically determine a histogram representing thedistribution of detected photons. Based on the histogram of photondistribution, the response of tissue to light pulses may be determinedto determine neural activity and/or other attributes of the brain.

A photodetector capable of detecting a single photon (i.e., a singleparticle of optical energy) is an example of a non-invasive detectorthat can be used in an optical measurement system to detect neuralactivity within the brain. An exemplary photodetector is implemented bya semiconductor-based single-photon avalanche diode (SPAD), which iscapable of capturing individual photons with very high time-of-arrivalresolution (a few tens of picoseconds).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments and are a partof the specification. The illustrated embodiments are merely examplesand do not limit the scope of the disclosure. Throughout the drawings,identical or similar reference numbers designate identical or similarelements.

FIG. 1 shows an exemplary optical measurement system.

FIG. 2 illustrates an exemplary detector architecture.

FIG. 3 illustrates an exemplary timing diagram for performing an opticalmeasurement operation using an optical measurement system.

FIG. 4 illustrates a graph of an exemplary temporal point spreadfunction that may be generated by an optical measurement system inresponse to a light pulse.

FIG. 5 shows an exemplary non-invasive wearable brain interface system.

FIG. 6 shows an exemplary wearable module assembly.

FIG. 7 shows an exemplary timing diagram.

FIGS. 8-9 illustrate exemplary phase lock loop (PLL) circuit basedarchitectures.

FIG. 10 shows an exemplary timing diagram.

FIGS. 11-13 illustrate exemplary PLL circuit based architectures.

FIG. 14 shows an exemplary timing diagram.

FIG. 15 shows a sequence of light pulses that may be applied to atarget.

FIG. 16 illustrates an exemplary PLL circuit based architecture.

FIGS. 17A-17B illustrate an example of aligning a measurement timewindow.

FIGS. 18-23 illustrate embodiments of a wearable device that includeselements of the optical detection systems described herein.

FIG. 24 illustrates an exemplary computing device.

FIGS. 25-26 illustrate exemplary methods.

DETAILED DESCRIPTION

Systems, circuits, and methods for phase lock loop (PLL) circuit basedadjustment of a measurement time window in an optical measurement systemare described herein.

For example, an exemplary system may include a photodetector configuredto generate a photodetector output pulse when the photodetector detectsa photon from a light pulse having a light pulse time period, atime-to-digital converter (TDC) configured to monitor for the occurrenceof the photodetector output pulse during a measurement time window thatis within and shorter in duration than the light pulse time period, aPLL circuit for the TDC, and a precision timing circuit connected to thePLL circuit and configured to adjust, based on at least one signalgenerated within the PLL circuit, a temporal position of the measurementtime window within the light pulse time period.

In some examples, the light pulse is included in a sequence of lightpulses generated by a light source and each having the light pulse timeperiod. In these examples, the system may further include a measurementtime window management circuit configured to direct the precision timingcircuit to sweep the measurement time window across the light pulse timeperiod while the sequence of light pulses are being generated. Asdescribed herein, the sweeping may result in a temporal point spreadfunction (TPSF) being generated based on timestamp symbols recorded bythe TDC while the measurement time window is being swept. Themeasurement time window management circuit may be further configured todetermine a property of the TPSF and identify, based on the property ofthe TPSF, a temporal location within the light pulse time period (e.g.,a temporal position that corresponds to the property). The measurementtime window management circuit may be further configured to direct theprecision timing circuit to adjust the temporal position of themeasurement time window to align a particular time bin within themeasurement time window with the temporal location within the lightpulse time period.

The systems, circuits, and methods described herein conserve power andensure that a measurement time window is consistently placed withrespect to each light pulse that is generated over the course of time inwhich a TPSF is generated. This, in turn, ensures that the TPSF isaccurately generated (i.e., that the samples used to generate the TPSFare placed in the right time bins). These and other advantages andbenefits of the present systems, circuits, and methods are describedmore fully herein.

FIG. 1 shows an exemplary optical measurement system 100 configured toperform an optical measurement operation with respect to a body 102.Optical measurement system 100 may, in some examples, be portable and/orwearable by a user. Optical measurement systems that may be used inconnection with the embodiments described herein are described morefully in U.S. patent application Ser. No. 17/176,315, filed Feb. 16,2021; U.S. patent application Ser. No. 17/176,309, filed Feb. 16, 2021;U.S. patent application Ser. No. 17/176,460, filed Feb. 16, 2021; U.S.patent application Ser. No. 17/176,470, filed Feb. 16, 2021; U.S. patentapplication Ser. No. 17/176,487, filed Feb. 16, 2021; U.S. patentapplication Ser. No. 17/176,539, filed Feb. 16, 2021; U.S. patentapplication Ser. No. 17/176,560, filed Feb. 16, 2021; and U.S. patentapplication Ser. No. 17/176,466, filed Feb. 16, 2021, which applicationsare incorporated herein by reference in their entirety.

In some examples, optical measurement operations performed by opticalmeasurement system 100 are associated with a time domain-based opticalmeasurement technique. Example time domain-based optical measurementtechniques include, but are not limited to, TCSPC, time domain nearinfrared spectroscopy (TD-NIRS), time domain diffusive correlationspectroscopy (TD-DCS), and time domain Digital Optical Tomography(TD-DOT).

As shown, optical measurement system 100 includes a detector 104 thatincludes a plurality of individual photodetectors (e.g., photodetector106), a processor 108 coupled to detector 104, a light source 110, acontroller 112, and optical conduits 114 and 116 (e.g., light pipes).However, one or more of these components may not, in certainembodiments, be considered to be a part of optical measurement system100. For example, in implementations where optical measurement system100 is wearable by a user, processor 108 and/or controller 112 may insome embodiments be separate from optical measurement system 100 and notconfigured to be worn by the user.

Detector 104 may include any number of photodetectors 106 as may serve aparticular implementation, such as 2^(n) photodetectors (e.g., 256, 512,. . . , 16384, etc.), where n is an integer greater than or equal to one(e.g., 4, 5, 8, 10, 11, 14, etc.). Photodetectors 106 may be arranged inany suitable manner.

Photodetectors 106 may each be implemented by any suitable circuitconfigured to detect individual photons of light incident uponphotodetectors 106. For example, each photodetector 106 may beimplemented by a single photon avalanche diode (SPAD) circuit and/orother circuitry as may serve a particular implementation.

Processor 108 may be implemented by one or more physical processing(e.g., computing) devices. In some examples, processor 108 may executeinstructions (e.g., software) configured to perform one or more of theoperations described herein.

Light source 110 may be implemented by any suitable component configuredto generate and emit light. For example, light source 110 may beimplemented by one or more laser diodes. In some examples, the lightemitted by light source 110 is high coherence light (e.g., light thathas a coherence length of at least 5 centimeters) at a predeterminedcenter wavelength.

Light source 110 is controlled by controller 112, which may beimplemented by any suitable computing device (e.g., processor 108),integrated circuit, and/or combination of hardware and/or software asmay serve a particular implementation. In some examples, controller 112is configured to control light source 110 by turning light source 110 onand off and/or setting an intensity of light generated by light source110. Controller 112 may be manually operated by a user, or may beprogrammed to control light source 110 automatically.

Light emitted by light source 110 may travel via an optical conduit 114(e.g., a light pipe, a light guide, a waveguide, a single-mode opticalfiber, and/or or a multi-mode optical fiber) to body 102 of a subject.In cases where optical conduit 114 is implemented by a light guide, thelight guide may be spring loaded and/or have a cantilever mechanism toallow for conformably pressing the light guide firmly against body 102.

Body 102 may include any suitable turbid medium. For example, in someimplementations, body 102 is a head or any other body part of a human orother animal. Alternatively, body 102 may be a non-living object. Forillustrative purposes, it will be assumed in the examples providedherein that body 102 is a human head.

As indicated by arrow 120, the light emitted by light source 110 entersbody 102 at a first location 122 on body 102. To this end, a distal endof optical conduit 114 may be positioned at (e.g., right above orphysically attached to) first location 122 (e.g., to a scalp of thesubject). In some examples, the light may emerge from optical conduit114 and spread out to a certain spot size on body 102 to fall under apredetermined safety limit.

As shown, a proximal end of optical conduit 116 (e.g., a light pipe, asingle-mode optical fiber, and/or or a multi-mode optical fiber) ispositioned at (e.g., right above or physically attached to) outputlocation 126 on body 102. In this manner, optical conduit 116 maycollect light 124 as it exits body 102 at location 126 and carry thelight to detector 104. The light may pass through one or more lensesand/or other optical elements (not shown) that direct the light ontoeach of the photodetectors 106 included in detector 104.

Photodetectors 106 may be connected in parallel in detector 104. Anoutput of each of photodetectors 106 may be accumulated to generate anaccumulated output of detector 104. Processor 108 may receive theaccumulated output and determine, based on the accumulated output, atemporal distribution of photons detected by photodetectors 106.Processor 108 may then generate, based on the temporal distribution, ahistogram representing a light pulse response of a target (e.g., braintissue, blood flow, etc.) in body 102. Example embodiments ofaccumulated outputs are described herein.

FIG. 2 illustrates an exemplary detector architecture 200 that may beused in accordance with the systems and methods described herein. Asshown, architecture 200 includes a SPAD circuit 202 that implementsphotodetector 106, a control circuit 204, a time-to-digital converter(TDC) 206, and a signal processing circuit 208. Architecture 200 mayinclude additional or alternative components as may serve a particularimplementation.

In some examples, SPAD circuit 202 includes a SPAD and a fast gatingcircuit configured to operate together to detect a photon incident uponthe SPAD. As described herein, SPAD circuit 202 may generate an outputwhen SPAD circuit 202 detects a photon.

The fast gating circuit included in SPAD circuit 202 may be implementedin any suitable manner. For example, the fast gating circuit may includea capacitor that is pre-charged with a bias voltage before a command isprovided to arm the SPAD. Gating the SPAD with a capacitor instead ofwith an active voltage source, such as is done in some conventional SPADarchitectures, has a number of advantages and benefits. For example, aSPAD that is gated with a capacitor may be armed practicallyinstantaneously compared to a SPAD that is gated with an active voltagesource. This is because the capacitor is already charged with the biasvoltage when a command is provided to arm the SPAD. This is describedmore fully in U.S. Pat. Nos. 10,158,038 and 10,424,683, incorporatedherein by reference in their respective entireties.

In some alternative configurations, SPAD circuit 202 does not include afast gating circuit. In these configurations, the SPAD included in SPADcircuit 202 may be gated in any suitable manner or be configured tooperate in a free running mode with passive quenching.

Control circuit 204 may be implemented by an application specificintegrated circuit (ASIC) or any other suitable circuit configured tocontrol an operation of various components within SPAD circuit 202. Forexample, control circuit 204 may output control logic that puts the SPADincluded in SPAD circuit 202 in either an armed or a disarmed state.

In some examples, control circuit 204 may control a gate delay, whichspecifies a predetermined amount of time control circuit 204 is to waitafter an occurrence of a light pulse (e.g., a laser pulse) to put theSPAD in the armed state. To this end, control circuit 204 may receivelight pulse timing information, which indicates a time at which a lightpulse occurs (e.g., a time at which the light pulse is applied to body102). Control circuit 204 may also control a programmable gate width,which specifies how long the SPAD is kept in the armed state beforebeing disarmed.

Control circuit 204 is further configured to control signal processingcircuit 208. For example, control circuit 204 may provide histogramparameters (e.g., time bins, number of light pulses, type of histogram,etc.) to signal processing circuit 208. Signal processing circuit 208may generate histogram data in accordance with the histogram parameters.In some examples, control circuit 204 is at least partially implementedby controller 112.

TDC 206 is configured to measure a time difference between an occurrenceof an output pulse generated by SPAD circuit 202 and an occurrence of alight pulse. To this end, TDC 206 may also receive the same light pulsetiming information that control circuit 204 receives. TDC 206 may beimplemented by any suitable circuitry as may serve a particularimplementation.

Signal processing circuit 208 is configured to perform one or moresignal processing operations on data output by TDC 206. For example,signal processing circuit 208 may generate histogram data based on thedata output by TDC 206 and in accordance with histogram parametersprovided by control circuit 204. To illustrate, signal processingcircuit 208 may generate, store, transmit, compress, analyze, decode,and/or otherwise process histograms based on the data output by TDC 206.In some examples, signal processing circuit 208 may provide processeddata to control circuit 204, which may use the processed data in anysuitable manner. In some examples, signal processing circuit 208 is atleast partially implemented by processor 108.

In some examples, each photodetector 106 (e.g., SPAD circuit 202) mayhave a dedicated TDC 206 associated therewith. For example, for an arrayof N photodetectors 106, there may be a corresponding array of N TDCs206. Alternatively, a single TDC 206 may be associated with multiplephotodetectors 106. Likewise, a single control circuit 204 and a singlesignal processing circuit 208 may be provided for a one or morephotodetectors 106 and/or TDCs 206.

FIG. 3 illustrates an exemplary timing diagram 300 for performing anoptical measurement operation using optical measurement system 100.Optical measurement system 100 may be configured to perform the opticalmeasurement operation by directing light pulses (e.g., laser pulses)toward a target within a body (e.g., body 102). The light pulses may beshort (e.g., 10-2000 picoseconds (ps)) and repeated at a high frequency(e.g., between 100,000 hertz (Hz) and 100 megahertz (MHz)). The lightpulses may be scattered by the target and then detected by opticalmeasurement system 100. Optical measurement system 100 may measure atime relative to the light pulse for each detected photon. By countingthe number of photons detected at each time relative to each light pulserepeated over a plurality of light pulses, optical measurement system100 may generate a histogram that represents a light pulse response ofthe target (e.g., a temporal point spread function (TPSF)). The termshistogram and TPSF are used interchangeably herein to refer to a lightpulse response of a target.

For example, timing diagram 300 shows a sequence of light pulses 302(e.g., light pulses 302-1 and 302-2) that may be applied to the target(e.g., tissue within a brain of a user, blood flow, a fluorescentmaterial used as a probe in a body of a user, etc.). Timing diagram 300also shows a pulse wave 304 representing predetermined gated timewindows (also referred as gated time periods) during whichphotodetectors 106 are gated ON to detect photons. Referring to lightpulse 302-1, light pulse 302-1 is applied at a time t₀. At a time t₁, afirst instance of the predetermined gated time window begins.Photodetectors 106 may be armed at time t₁, enabling photodetectors 106to detect photons scattered by the target during the predetermined gatedtime window. In this example, time t₁ is set to be at a certain timeafter time t₀, which may minimize photons detected directly from thelaser pulse, before the laser pulse reaches the target. However, in somealternative examples, time t₁ is set to be equal to time t₀.

At a time t₂, the predetermined gated time window ends. In someexamples, photodetectors 106 may be disarmed at time t₂. In otherexamples, photodetectors 106 may be reset (e.g., disarmed and re-armed)at time t₂ or at a time subsequent to time t₂. During the predeterminedgated time window, photodetectors 106 may detect photons scattered bythe target. Photodetectors 106 may be configured to remain armed duringthe predetermined gated time window such that photodetectors 106maintain an output upon detecting a photon during the predeterminedgated time window. For example, a photodetector 106 may detect a photonat a time t₃, which is during the predetermined gated time windowbetween times t₁ and t₂. The photodetector 106 may be configured toprovide an output indicating that the photodetector 106 has detected aphoton. The photodetector 106 may be configured to continue providingthe output until time t₂, when the photodetector may be disarmed and/orreset. Optical measurement system 100 may generate an accumulated outputfrom the plurality of photodetectors. Optical measurement system 100 maysample the accumulated output to determine times at which photons aredetected by photodetectors 106 to generate a TPSF.

As mentioned, in some alternative examples, photodetector 106 may beconfigured to operate in a free-running mode such that photodetector 106is not actively armed and disarmed (e.g., at the end of eachpredetermined gated time window represented by pulse wave 304). Incontrast, while operating in the free-running mode, photodetector 106may be configured to reset within a configurable time period after anoccurrence of a photon detection event (i.e., after photodetector 106detects a photon) and immediately begin detecting new photons. However,only photons detected within a desired time window (e.g., during eachgated time window represented by pulse wave 304) may be included in theTPSF.

FIG. 4 illustrates a graph 400 of an exemplary TPSF 402 that may begenerated by optical measurement system 100 in response to a light pulse404 (which, in practice, represents a plurality of light pulses). Graph400 shows a normalized count of photons on a y-axis and time bins on anx-axis. As shown, TPSF 402 is delayed with respect to a temporaloccurrence of light pulse 404. In some examples, the number of photonsdetected in each time bin subsequent to each occurrence of light pulse404 may be aggregated (e.g., integrated) to generate TPSF 402. TPSF 402may be analyzed and/or processed in any suitable manner to determine orinfer detected neural activity.

Optical measurement system 100 may be implemented by or included in anysuitable device. For example, optical measurement system 100 may beincluded, in whole or in part, in a non-invasive wearable device (e.g.,a headpiece) that a user may wear to perform one or more diagnostic,imaging, analytical, and/or consumer-related operations. Thenon-invasive wearable device may be placed on a user's head or otherpart of the user to detect neural activity. In some examples, suchneural activity may be used to make behavioral and mental stateanalysis, awareness and predictions for the user.

Mental state described herein refers to the measured neural activityrelated to physiological brain states and/or mental brain states, e.g.,joy, excitement, relaxation, surprise, fear, stress, anxiety, sadness,anger, disgust, contempt, contentment, calmness, focus, attention,approval, creativity, positive or negative reflections/attitude onexperiences or the use of objects, etc. Further details on the methodsand systems related to a predicted brain state, behavior, preferences,or attitude of the user, and the creation, training, and use of neuromescan be found in U.S. Provisional Patent Application No. 63/047,991,filed Jul. 3, 2020. Exemplary measurement systems and methods usingbiofeedback for awareness and modulation of mental state are describedin more detail in U.S. patent application Ser. No. 16/364,338, filedMar. 26, 2019, published as US2020/0196932A1. Exemplary measurementsystems and methods used for detecting and modulating the mental stateof a user using entertainment selections, e.g., music, film/video, aredescribed in more detail in U.S. patent application Ser. No. 16/835,972,filed Mar. 31, 2020, published as US2020/0315510A1. Exemplarymeasurement systems and methods used for detecting and modulating themental state of a user using product formulation from, e.g., beverages,food, selective food/drink ingredients, fragrances, and assessment basedon product-elicited brain state measurements are described in moredetail in U.S. patent application Ser. No. 16/853,614, filed Apr. 20,2020, published as US2020/0337624A1. Exemplary measurement systems andmethods used for detecting and modulating the mental state of a userthrough awareness of priming effects are described in more detail inU.S. patent application Ser. No. 16/885,596, filed May 28, 2020,published as US2020/0390358A1. These applications and corresponding U.S.publications are incorporated herein by reference in their entirety.

FIG. 5 shows an exemplary non-invasive wearable brain interface system500 (“brain interface system 500”) that implements optical measurementsystem 100 (shown in FIG. 1 ). As shown, brain interface system 500includes a head-mountable component 502 configured to be attached to auser's head. Head-mountable component 502 may be implemented by a capshape that is worn on a head of a user. Alternative implementations ofhead-mountable component 502 include helmets, beanies, headbands, otherhat shapes, or other forms conformable to be worn on a user's head, etc.Head-mountable component 502 may be made out of any suitable cloth, softpolymer, plastic, hard shell, and/or any other suitable material as mayserve a particular implementation. Examples of headgears used withwearable brain interface systems are described more fully in U.S. Pat.No. 10,340,408, incorporated herein by reference in its entirety.

Head-mountable component 502 includes a plurality of detectors 504,which may implement or be similar to detector 104, and a plurality oflight sources 506, which may be implemented by or be similar to lightsource 110. It will be recognized that in some alternative embodiments,head-mountable component 502 may include a single detector 504 and/or asingle light source 506.

Brain interface system 500 may be used for controlling an optical pathto the brain and for transforming photodetector measurements into anintensity value that represents an optical property of a target withinthe brain. Brain interface system 500 allows optical detection of deepanatomical locations beyond skin and bone (e.g., skull) by extractingdata from photons originating from light source 506 and emitted to atarget location within the user's brain, in contrast to conventionalimaging systems and methods (e.g., optical coherence tomography (OCT)),which only image superficial tissue structures or through opticallytransparent structures.

Brain interface system 500 may further include a processor 508configured to communicate with (e.g., control and/or receive signalsfrom) detectors 504 and light sources 506 by way of a communication link510. Communication link 510 may include any suitable wired and/orwireless communication link. Processor 508 may include any suitablehousing and may be located on the user's scalp, neck, shoulders, chest,or arm, as may be desirable. In some variations, processor 508 may beintegrated in the same assembly housing as detectors 504 and lightsources 506.

As shown, brain interface system 500 may optionally include a remoteprocessor 512 in communication with processor 508. For example, remoteprocessor 512 may store measured data from detectors 504 and/orprocessor 508 from previous detection sessions and/or from multiplebrain interface systems (not shown). Power for detectors 504, lightsources 506, and/or processor 508 may be provided via a wearable battery(not shown). In some examples, processor 508 and the battery may beenclosed in a single housing, and wires carrying power signals fromprocessor 508 and the battery may extend to detectors 504 and lightsources 506. Alternatively, power may be provided wirelessly (e.g., byinduction).

In some alternative embodiments, head mountable component 502 does notinclude individual light sources. Instead, a light source configured togenerate the light that is detected by detector 504 may be includedelsewhere in brain interface system 500. For example, a light source maybe included in processor 508 and coupled to head mountable component 502through optical connections.

Each of the light sources described herein may be implemented by anysuitable device. For example, a light source as used herein may be, forexample, a distributed feedback (DFB) laser, a super luminescent diode(SLD), a light emitting diode (LED), a diode-pumped solid-state (DPSS)laser, a laser diode (LD), a super luminescent light emitting diode(sLED), a vertical-cavity surface-emitting laser (VCSEL), a titaniumsapphire laser, a micro light emitting diode (mLED), and/or any othersuitable laser or light source.

Optical measurement system 100 may alternatively be included in anon-wearable device (e.g., a medical device and/or consumer device thatis placed near the head or other body part of a user to perform one ormore diagnostic, imaging, and/or consumer-related operations). Opticalmeasurement system 100 may alternatively be included in a sub-assemblyenclosure of a wearable invasive device (e.g., an implantable medicaldevice for brain recording and imaging).

Optical measurement system 100 may be modular in that one or morecomponents of optical measurement system 100 may be removed, changedout, or otherwise modified as may serve a particular implementation.Additionally or alternatively, optical measurement system 100 may bemodular such that one or more components of optical measurement system100 may be housed in a separate housing (e.g., module) and/or may bemovable relative to other components. Exemplary modular multimodalmeasurement systems are described in more detail in U.S. patentapplication Ser. No. 17/176,460, filed Feb. 16, 2021, U.S. patentapplication Ser. No. 17/176,470, filed Feb. 16, 2021, U.S. patentapplication Ser. No. 17/176,487, filed Feb. 16, 2021, U.S. ProvisionalPatent Application No. 63/038,481, filed Jun. 12, 2020, and U.S. patentapplication Ser. No. 17/176,560, filed Feb. 16, 2021, which applicationsare incorporated herein by reference in their respective entireties.

To illustrate, FIG. 6 shows an exemplary wearable module assembly 600(“assembly 600”) that implements one or more of the optical measurementfeatures described herein. Assembly 600 may be worn on the head or anyother suitable body part of the user. As shown, assembly 600 may includea plurality of modules 602 (e.g., modules 602-1 through 602-3). Whilethree modules 602 are shown to be included in assembly 600 in FIG. 6 ,in alternative configurations, any number of modules 602 (e.g., a singlemodule up to sixteen or more modules) may be included in assembly 600.Moreover, while modules 602 are shown to be adjacent to and touching oneanother, modules 602 may alternatively be spaced apart from one another(e.g., in implementations where modules 602 are configured to beinserted into individual slots or cutouts of the headgear).

Each module 602 includes a source 604 and a plurality of detectors 606(e.g., detectors 606-1 through 606-6). Source 604 may be implemented byone or more light sources similar to light source 110. Each detector 606may implement or be similar to detector 104 and may include a pluralityof photodetectors (e.g., SPADs) as well as other circuitry (e.g., TDCs).As shown, detectors 606 are arranged around and substantiallyequidistant from source 604. In other words, the spacing between a lightsource (i.e., a distal end portion of a light source optical conduit)and the detectors (i.e., distal end portions of optical conduits foreach detector) are maintained at the same fixed distance on each moduleto ensure homogeneous coverage over specific areas and to facilitateprocessing of the detected signals. The fixed spacing also providesconsistent spatial (lateral and depth) resolution across the target areaof interest, e.g., brain tissue. Moreover, maintaining a known distancebetween the light emitter and the detector allows subsequent processingof the detected signals to infer spatial (e.g., depth localization,inverse modeling) information about the detected signals. Detectors 606may be alternatively disposed as may serve a particular implementation.

FIG. 7 shows an exemplary timing diagram 700 of a number of pulsedsignals that may be provided in optical measurement system 100 toaccurately capture a temporal point spread function (TPSF) from adiffuse medium. Signal 1 represents a first pulsed light signal that maybe applied to a target, signal 2 represents a second pulsed light signalthat may be applied to the target, and signal 3 represents a pulsedgating signal that may be used to specify a time period (referred to ast_gate) during which a SPAD is ON (i.e., armed) to detect a photon frompulses included in the first and second pulsed light signals after theyare scattered by the target. An exemplary time between the rising edgesof a pulse in the first and the second light pulse signals is t_light(e.g., around 25 ns if the light repetition rate is 40 MHz). Some of thesystems, circuits, and methods described herein may be used to preciselyspecify a duration and temporal position of the gate pulses included inthe gating signal with respect to the light pulses. Some of the systems,circuits, and methods described herein may additionally or alternativelybe used to precisely specify a duration and temporal position of variousother pulses used within optical measurement system 100.

FIG. 8 illustrates an exemplary PLL circuit based architecture 800 thatmay be included within optical measurement system 100 to generate andset a temporal position (e.g., of a rising edge and/or of a fallingedge) of a timing pulse. As shown, architecture 800 includes a PLLcircuit 802 communicatively coupled to a precision timing circuit 804.PLL circuit 802 includes a VCO 806, a feedback divider 808, a phasedetector 810, a charge pump 812, and a loop filter 814 connected in afeedback loop configuration. Phase detector 810 may receive a referenceclock as an input such that PLL circuit 802 has a PLL feedback perioddefined by the reference clock. The reference clock may have anysuitable frequency, such as any frequency between 1 MHz and 200 MHz.

VCO 806 may be implemented by any suitable combination of circuitry(e.g., a differential multi-stage gated ring oscillator (GRO) circuit)and is configured to lock to the reference clock (i.e., to a multiple ofa frequency of the reference clock). To that end, VCO 806 may include aplurality of stages configured to output a plurality of fine phasesignals each having a different phase and uniformly distributed in time.In some examples, each stage may output two fine phase signals that havecomplimentary phases. VCO 806 may include any suitable number of stagesconfigured to output any suitable number of fine phase signals (e.g.,eight stages that output sixteen fine phase signals). The duration of afine phase signal pulse depends on the oscillator frequency of VCO 806and the total number of fine phase signals. For example, if theoscillator frequency is 1 gigahertz (GHz) and the total number of finephase signals is sixteen, the duration of a pulse included in a finephase signal is 1 GHz/16, which is 62.5 picoseconds (ps). As describedherein, these fine phase signals may provide precision timing circuit804 with the ability to adjust a phase (i.e., temporal position) of atiming pulse with relatively fine resolution.

Feedback divider 808 is configured to be clocked by a single fine phasesignal included in the plurality of fine phase signals output by VCO 806and have a plurality of feedback divider states during the PLL feedbackperiod. The number of feedback divider states depends on the oscillatorfrequency of VCO 806 and the frequency of the reference clock. Forexample, if the oscillator frequency is 1 gigahertz (GHz) and thereference clock has a frequency of 50 MHz, the number of feedbackdivider states is equal to 1 GHz/50 MHz, which is equal to 20 feedbackdivider states. As described herein, these feedback divider states mayprovide precision timing circuit 804 with the ability to adjust a phase(i.e., temporal position) of a timing pulse with relatively courseresolution.

Feedback divider 808 may be implemented by any suitable circuitry. Insome alternative examples, feedback divider 808 is at least partiallyintegrated into precision timing circuit 804.

As shown, the fine phase signals output by VCO 806 and state information(e.g., signals and/or data) representative of the feedback dividerstates within feedback divider 808 are input into precision timingcircuit 804. Precision timing circuit 804 may be configured to generatea timing pulse and set, based on a combination of one of the fine phasesignals and one of the feedback dividers states, a temporal position ofthe timing pulse within the PLL feedback period. For example, if thereare N total fine phase signals and M total feedback divider states,precision timing circuit 804 may set the temporal position of the timingpulse to be one of N times M possible temporal positions within the PLLfeedback period. To illustrate, if N is 16 and M is 20, and if theduration of a pulse included in a fine phase signal is 62.5 ps, thetemporal position of the timing pulse may be set to be one of 320possible positions in 62.5 ps steps.

The timing pulse generated by precision timing circuit 804 may be usedwithin optical measurement system 100 in any suitable manner. Forexample, the timing pulse may be configured to trigger a start (e.g., arising edge) of an output pulse used by a component within opticalmeasurement system 100. Alternatively, the timing pulse may beconfigured to trigger an end (e.g., a falling edge) of an output pulseused by a component within optical measurement system 100.Alternatively, the timing pulse itself may be provided for use as anoutput pulse used by a component within optical measurement system 100.In some examples, precision timing circuit 804 may generate multipletiming pulses each used for a different purpose within opticalmeasurement system 100. These examples are each described in more detailherein.

FIG. 9 shows an exemplary implementation 900 of PLL circuit basedarchitecture 800. In implementation 900, feedback divider 808 isimplemented by a linear feedback shift register (LFSR) 902 and precisiontiming circuit 804 is implemented by a quadrature clock block 904, aplurality of phase intersection blocks (e.g., phase intersection block906-1 through 906-3), and various other electrical components (e.g., abuffer 908, a multiplexer 910, a gate 912, and a register 914). FIG. 9also depicts a call out 916 that shows exemplary logic included in eachphase intersection block 906.

In implementation 900, VCO 806 is configured to output sixteen finephase signals (labeled “fine” in FIG. 9 ). The fine phase signals arebuffered by buffer 908 and input into quadrature clock block 904 andeach phase intersection block 906.

Quadrature clock block 904 is configured to select, from the pluralityof fine phase signals generated by VCO 806, four fine phase signals thatare quadrature shifted from each other (e.g., evenly spaced at arelative 0, 90, 180, and 270 degrees) for use as quadrature clocksignals. One of the quadrature clock signals (e.g., quadrature phase 0)is used to clock LFSR 902. The quadrature clock signals are also eachprovided to each phase intersection block 906.

LFSR 902 is configured to have a plurality of feedback divider statesduring each PLL feedback period, as described herein. LFSR 902 outputsstate information (labeled “coarse” in FIG. 9 ) by, for example,counting up to the total number of feedback dividers states once per PLLfeedback period. This course count is provided to each phaseintersection block 906.

LFSR 902 is further configured to generate a load signal, which occurseach time LFSR 902 wraps. The load signal and an output of phaseintersection block 906-1 are input into a multiplexer 910, which selectsone of the signals for use as the feedback signal (pll_feedback) that isprovided to phase detector 810. This will be described in more detailbelow.

Each of phase intersection blocks 906 can be independently programmed togenerate a single pulse (i.e., a timing pulse) that is the intersectionor combination of a chosen coarse state (i.e., a chosen feedback dividerstate), and a chosen fine signal phase (i.e., a phase of a chosen finephase signal). It will be recognized that any number of phaseintersection blocks 906 may be included in precision timing circuit 804to generate any number of timing pulses as may serve a particularimplementation.

Once per PLL feedback period, the feedback divider state matches theprogrammed target, generating a combinational match signal. Thequadrature clock signals are used to register and delay thecombinational match signal. In this way, four match signals aregenerated, which are quadrature shifted from each other and which eachoccur once per PLL feedback period. Inside each phase intersection block906, one of the selected fine phases is logically ANDed with one of theregistered match signals, resulting in a single output pulse per PLLfeedback period. The temporal position of this output pulse can beselected with a granularity of the VCO stage. For example, if the PLLcircuit is locked to a VCO oscillator frequency of 1 GHz and each of theeight VCO oscillator stages has a 62.5 ps delay, and the PLL referenceclock is 50 MHz (feedback period is 20 ns), then the feedback dividerhas 20 states (coarse), the ring oscillator has 16 states (fine), andthe temporal position of the timing pulse output by the phaseintersection block 906 can be programmed to be any of 20*16=320 possiblepositions in 62.5 ps steps.

To illustrate, with respect to the phase intersection block 906 shown incall out 916, phase intersection block 906 may be configured receive thefollowing inputs: the plurality of fine phase signals (labeled “fine”),the quadrature clock signals (labeled “clocks”) output by quadratureclock block 904, a programmable target state signal (labeled“Ifsr_target”) that identifies a target feedback divider state includedin the plurality of feedback divider states of LFSR 902, and aprogrammable target fine phase signal (labeled “sel_fine”) identifying atarget fine phase signal included in the plurality of fine phase signalsand that, in combination with the target feedback divider state, resultsin the timing pulse output by phase intersection block 906 occurring ata desired temporal position.

As illustrated by comparison block 918, phase intersection block 906 isconfigured to generate a combination match signal (labeled “match_comb”)when a current feedback divider state (e.g., a particular feedbackdivider state count) matches the target feedback state. Phaseintersection block 906 may use the quadrature clock signals (e.g., byinputting them into registers 920) to generate four registered matchsignals representative of the combination match signal. These fourregistered match signals are represented by match_0, match_90,match_180, and match_270 are quadrature shifted from each other. Thefour registered match signals are input into a multiplexer 922, whichreceives a selector input labeled sel_dly that selects a particularmatch signal from the four registered match signal that is aligned(e.g., most aligned) with a pulse included in the target fine phasesignal. The selected match signal and the target fine phase signal (asoutput by a multiplexer 924 controlled by a selector signal labeledsel_fine) into an AND gate 926 to output the timing pulse at thetemporal position.

The timing pulse of a single phase intersection block 906 may be arelatively narrow pulse. For example, the timing pulse may have a pulsewidth of half of the VCO oscillator period. In this example, if the VCOoscillator frequency is 1 GHz (i.e., the period is 1 ns), the timingpulse width will be 500 ps. For some applications, a different pulsewidth for a particular signal (e.g., a gating signal) is desirable. Inthis case, two phase intersection blocks (e.g., phase intersectionblocks 906-2 and 906-3) may be combined and used to trigger the startand end of an output pulse used by a component in optical measurementsystem 100. For example, in the example of FIG. 9 , phase intersectionblock 906-3 is configured to output a timing pulse labeled en_start,which is configured to trigger a start of an output pulse labeled en.Phase intersection block 906-2 is configured to output a timing pulselabeled en_stop, which is configured to trigger an end of the outputpulse labeled en. The temporal positions of the timing pulses may be setas described herein to specify a duration of the output pulse. In theexample of FIG. 9 , at the programmed time, the rise of en_start causesa register to output a logic 1 to the signal named en. At a differentprogrammed time, the rise of the gate_stop signal resets the register tologic 0.

As mentioned, the load signal generated by LFSR 902 and an output signalof phase intersection block 906-1 are input into multiplexer 910, whichselects one of the signals for use as the feedback signal (pll_feedback)that is provided to phase detector 810. By using the output signal(which has a programmable phase as described herein) of phaseintersection block 906-1 as the PLL feedback signal (pll_feedback), thephase of the PLL feedback signal shifts the position of all othersignals (e.g., timing pulses) generated by the phase intersection blocks906 and other circuitry (e.g., timestamp generation circuitry, asdescribed herein) connected to the PLL circuit. This is because the PLLfeedback signal is part of the PLL control loop. The PLL will adjust theVCO oscillator phase and frequency until the reference clock (REFCLK)and PLL feedback signals are aligned (typically only positive ornegative edges are aligned since pulse widths are often different). Ifthe PLL control loop advances the phase of the PLL feedback signal tocause it to align in time with the REFCLK signal, then the other phaseintersection block outputs advance as well. This provides an absolutephase reference to the REFCLK signal. The individual phase intersectionblocks 906 can be independently programmed, which allows for anyrelative spacing between the timing pulses within the PLL feedbackperiod. This configuration gives precision and flexibility.

FIG. 10 shows a timing diagram 1000 showing a PLL reference clock(Reference), fine phase signals generated by VCO 806, and arepresentation of the PLL feedback divider state (fbdiv). In thisexample, feedback divider 808 counts from 20 down to 1 and repeats. Eachfeedback divider state occurs only once per reference period, thusproviding a coarse position that is unique within the reference period.Within each feedback divider state, VCO 806 cycles through its states(sixteen in this example), thus providing a fine position that is notunique within the reference period. The intersection of a particularcoarse state and fine state gives a precise, unique position within thereference period.

As shown, the beginning of an output signal (en_start) is triggered whenfeedback state is 19 (sel_fbdiv) through a cascade of events. Acombinational equality comparison of fbdiv and sel_fbdiv (named coarseand Ifsr_target in FIG. 9 ) gives a combinational signal,match_comb=(fbdiv==sel_fbdiv). This combinational match signal is thensampled by quadrature clocks giving clean quadrature-shifted matchsignals at relative 0, 90, 180, and 270 degrees. A multiplexer (e.g.,multiplexer 922) selects one of these four match signals, chosen tocenter the fine phase within the coarse match signal. Anothermultiplexer (e.g., multiplexer 924) selects one of the sixteen finephase signals. These two signals are then ANDed together (e.g., with ANDgate 926). When properly aligned, this circuit can generate a singletiming pulse anywhere within the reference period, with the same spacingresolution as the VCO oscillator phase.

In timing diagram 1000, phase intersection block 906-3 generatesen_start, which marks the beginning of the en pulse. Phase intersectionblock 906-2 generates en_stop, which marks the end of the en pulse. Inthis way, a precise, repeatable output signal (en) is generated.

As mentioned, a timing pulse generated by precision timing circuit 804may be used within optical measurement system 100 in any suitablemanner. For example, a timing pulse generated by precision timingcircuit 804 or an output pulse having a start time or an end timedefined by the temporal position of the timing pulse may be configuredto be used as a gate pulse configured to trigger the arming anddisarming of a photodetector (e.g., a SPAD).

Additionally or alternatively, a timing pulse generated by precisiontiming circuit 804 or an output pulse having a start time or an end timedefined by the temporal position of the timing pulse may be configuredto be used as a calibration pulse for one or more TDCs or anothercomponent of optical measurement system 100.

Additionally or alternatively, a timing pulse generated by precisiontiming circuit 804 or an output pulse having a start time or an end timedefined by the temporal position of the timing pulse may be configuredto be used to trigger a light source to output a light pulse.

In some examples, precision timing circuit 804 may be configured togenerate a sequence of timing pulses each configured to have the sametemporal position within the PLL feedback period. As the sequence isbeing generated, precision timing circuit 804 may receive a command toadjust the temporal position of the timing pulses within the PLLfeedback period. The command may be provided by a user of opticalmeasurement system 100 or automatically by a component within opticalmeasurement system 100 without input being provided by a user of opticalmeasurement system 100. In response to receiving the command, precisiontiming circuit 804 may adjust the temporal position by selecting adifferent fine phase signal and/or feedback divider state to be used asthe combination that sets the temporal position. Based on the updatedcombination, precision timing circuit 804 may adjust the temporalposition of subsequent timing pulses that are generated.

FIG. 11 illustrates an exemplary PLL circuit based architecture 1100that may be configured to generate and distribute a timestamp signal busto one or more TDCs (e.g., to each TDC included in an array of TDCs)included in optical measurement system 100. As shown, architecture 1100includes the PLL circuit 802 described in connection with FIG. 8communicatively coupled to a timestamp generation circuit 1002.Timestamp generation circuit 1002 may be configured to generate, basedon a subset of the fine phase signals that define a plurality of finestates for the plurality of fine phase signals, a timestamp signal busrepresentative of a plurality of timestamp symbols. Timestamp generationcircuit 1002 may be further configured to transmit the timestamp signalbus to one or more TDCs. These operations are described in more detailherein.

In some examples, PLL circuit based architectures 800 and 1100 may becombined to form a PLL circuit based architecture that includes bothprecision timing and timestamp generation functionality. For example,FIG. 12 shows an exemplary PLL circuit based architecture 1200 thatincludes both precision timing circuit 804 and timestamp generationcircuit 1002. PLL circuit based architecture 1200 will be used in theexamples provided herein.

FIG. 13 shows an exemplary implementation 1300 of PLL circuit basedarchitecture 1200. Implementation 1300 is similar to implementation 900,except that implementation 1300 also includes a timestamp generationcircuit 1002. FIG. 13 depicts a call out 1302 that shows exemplary logicincluded in timestamp generation circuit 1002.

Timestamp generation circuit 1002 is configured to use the fine phasesignals and the load signal to generate a timestamp signal busrepresentative of a plurality of timestamp symbols. The timestamp signalbus may be generated centrally and then distributed to one or more TDCs(e.g., across a chip that implements one or more detectors).

A TDC may use the timestamp signal bus to generate a timestamp thatcorresponds to which a photodetector output pulse is detected by theTDC, thereby indicating an arrival time of a photon detected by aphotodetector. For example, as the timestamp signal bus is beingprovided to the TDC (i.e., as a sequence of timestamp symbols is beingdelivered to the TDC), the TDC may record a whatever timestamp symbol ispresent at the TDC when the photodetector output pulse occurs.

Hence, a measurement time window during which a TDC (or a plurality ofTDCs) monitors for an occurrence of a photodetector output pulse may bedefined by a sweep of a plurality of timestamp symbols that occur duringa PLL feedback period. For example, if 128 timestamp symbols areincluded in the timestamp signal bus per PLL feedback period, themeasurement time window may have a duration equal to a duration of the128 timestamp symbols.

Moreover, a TPSF that is generated based on the recorded timestampsymbols output by one or more TDCs may have a temporal resolution thatcorresponds to (e.g., that is equal to) the number of timestamp symbolsincluded in a PLL feedback period. For example, if 128 timestamp symbolsare included in the timestamp signal bus per PLL feedback period, theTPSF may have a temporal resolution of 128 time bins.

Skew across a conventional timestamp signal bus can cause one or moretimestamp symbols in the signal bus to be misinterpreted. Hence,timestamp generation circuit 1002 builds some redundancy into thetimestamp signal bus to create a timestamp that is more robust againstskew.

For example, as shown, timestamp generation circuit 1002 may include acourse counter 1304 configured to be clocked by one of the plurality offine phase signals and receive a load signal generated by LFSR 902. Theload signal is configured to reset course counter 1304 at a beginning ofeach PLL feedback period. Course counter 1304 is configured to output acourse count signal (course_count) comprising a course count up to amaximum value associated with the course counter 1304. Timestampgeneration circuit 1002 further includes a first register 1306-1configured to sample the course count signal to generate a course earlysignal (course_early) and a second register 1306-2 configured to samplethe course early signal to generate a course late signal (course_late).The timestamp signal bus output by timestamp generation circuit 1002includes a subset of fine phase signals, the course early signal, andthe course late signal.

To illustrate, the fine phase signals included in the timestamp signalmay be a selection of eight of the sixteen fine phase signals output byVCO 806. Only one of these eight signals changes at a time, and theytogether are sufficient to uniquely identify sixteen states. The courseearly and course late signals are both representations of the coarsecounter output, but shifted relative to each other such that at alltimes. In this manner, one of the two coarse buses will be stable. Thisredundancy in the coarse bus, along with the proper alignment betweenthe fine and coarse, are what provide robustness against distributionskew of the timestamp signal bus.

Course counter 1304 is reset each time the PLL feedback divider loadsignal is asserted. This keeps the timestamp signal bus synchronizedrelative to the PLL feedback signal. If this timestamp active region(the region in which it is desirable to have a timestamp) is of shorterduration than the PLL feedback period, then the course counter width canbe smaller than the PLL feedback divider width. However, in this case,course counter 1304 must saturate at its maximum count until the loadsignal is asserted rather than roll over. This design may ensure thatany valid timestamp symbol only occurs once within the PLL feedbackperiod. For the case where course counter 1304 saturates, this alsomeans that any timestamps with the coarse counter 1304 at its saturatedvalue (e.g. 7 for a 3-bit counter) may be discarded.

The load signal resets the course counter 1304, then one of the sixteenfine phase signals is selected to clock course counter 1304. The outputof course counter 1304 may have some uncertainty or settling time, sothe counter output is sampled by register 1306-1 to create signalcoarse_early. The coarse_early signal is then resampled by register1306-2 to generate the coarse_late signal. The counter clock, clk_early,and clk_late signals may be independently chosen from among the sixteenfine signals to ensure proper alignment of the timestamp signal bus. Inthis example, proper clock phases are chosen to align the coarse_earlysignal such that its stable region is centered around fine phase signals0 through 7. Likewise, the coarse_late signal is aligned such that it isstable around fine phase signals 8 though 15.

FIG. 14 shows a timing diagram 1400 showing fine phase signals(vco_out), fine phase states (fine state), coarse counter state (coursecount), and proper alignment of timestamp sub-symbols (fine,coarse_early and coarse_late). Only eight of the vco_out signals arerequired to correctly identify all sixteen fine phase states (one signalfrom each of the complementary stages). In this example, the first eightvco_out signals (vco_out[7:0]) are used. The signal marked “fine state”is a representation of the fine phase state, from 0 to 15. A coarsecounter, clocked from one of the sixteen vco_out clock phases is resetto zero at the beginning of the repeating measurement interval (periodof PLL reference clock), and then counts up, but saturates at itsmaximum value (for a three bit binary counter it saturates at 7). Thiscoarse counter, which might have a long settling time (represented byXs), is then resampled twice to generate coarse_early and coarse latesignals. These resampled coarse sub-symbols have short settling time(represented by Xs), and are aligned precisely to maximize skew marginbetween fine, coarse_early, and coarse_late sub-symbols. This alignmentis shown with diagonal hash markings. The final timestamp signal bus isthe concatenation of coarse_late, coarse_early, and vco_out[7:0]. Inthis example, the timestamp signal bus is 3+3+8=14 bits wide. For symboldecode, first the fine state is decoded, if fine state is 0 to 7, thenthe coarse early is used as the coarse value. If fine state is 8 to 15,then coarse late is used.

A TDC that receives the timestamp signal bus may be configured to recorda timestamp symbol included in the timestamp signal bus when the TDCdetects an occurrence of a photodetector output pulse generated by aphotodetector, where the photodetector output pulse indicates that thephotodetector has detected a photon from the light pulse after a lightpulse is scattered by a target.

In some examples, signal processing circuit 208 may include logicconfigured to decode a recorded timestamp symbol into a timestamprepresentative of when the photodetector output pulse is received by theTDC. For example, signal processing circuit 208 may be configured toreceive a recorded timestamp symbol, and analyze the eight fine phasesignals to identify which of the sixteen possible fine phase states arerepresented. In some examples, signal processing circuit 208 may use abubble correction algorithms to do this. If the fine state is between 0and 7, signal processing circuit 208 may use the coarse_early value.Alternatively, if the fine state is between 8 and 15, signal processingcircuit 208 may use the coarse_late value.

FIG. 15 shows a sequence of light pulses (e.g., laser pulses) 1502-1through 1502-3 that may be applied to a target in accordance with thesystems, circuits, and methods described herein. While three lightpulses 1502 are shown in FIG. 15 , it will be recognized that any numberof light pulses 1502 may be included in the sequence of light pulsesapplied to the target as may serve a particular implementation.

As shown, each light pulse 1502 has a corresponding light pulse timeperiod. In the particular example of FIG. 15 , the light pulse timeperiod is the time between a rising edge of a light pulse (e.g., lightpulse 1502-1) and a rising edge of a subsequent light pulse (e.g., lightpulse 1502-2). The light pulse time period may be of any suitableduration (e.g., between 20-40 ns). As shown, each light pulse 1502 mayhave a relatively low duty cycle (i.e., each light pulse 1502 is only onfor a small portion of its corresponding light pulse time period).

As described herein, a photon of a light pulse (e.g., any of lightpulses 1502) may be detected by a photodetector after the light pulse isapplied to and scattered by a target. When the photon is detected by thephotodetector, the photodetector may output a photodetector outputpulse. A TDC (e.g., TDC 206) may be configured to monitor for thephotodetector output pulse during a measurement time window, asdescribed herein. When the TDC detects the photodetector output pulse,the TDC may record a timestamp symbol, which may be used to generate aTPSF, as described herein. In some examples, an array of TDCs may eachmonitor for photodetector output pulses during the same measurement timewindow. In these examples, the timestamp signals recorded by the TDCsmay all be used to generate the TPSF.

As shown, the measurement time window is within and shorter in durationthan the light pulse time period. This is because photons of interestfor a TPSF are not detected by the photodetector until a certain amountof time after the light pulse 1502 is applied and only occur within arelatively short amount of time. For example, an exemplary duration of ameasurement time window is between 2-5 ns. Hence, by minimizing theduration of the measurement time window (as opposed to the measurementtime window having a duration that spans the entire light pulse timeperiod), power and other resources within optical measurement system 100may be conserved.

The systems, circuits, and methods described herein are configured toadjust a temporal position of the measurement time window such that themeasurement time window is consistently placed with respect to eachlight pulse 1502. This ensures that the TPSF is accurately generated(i.e., that the samples used to generate the TPSF are placed in theright time bins).

FIG. 16 illustrates an exemplary PLL circuit based architecture 1600that may be included within optical measurement system 100 to adjust ameasurement time window during which a TDC monitors for an occurrence ofa photodetector output pulse. PLL circuit based architecture 1600 issimilar to PLL circuit based architecture 1200, except that PLL circuitbased architecture 1600 further includes a measurement time windowmanagement circuit 1602 (“management circuit 1602”) communicativelycoupled to precision timing circuit 804.

Management circuit 1602 may be implemented by any suitable combinationof components. For example, management circuit 1602 may be implementedby a memory storing instructions and a processor communicatively coupledto the memory and configured to execute the instructions to perform anyof the management circuit-related operations described herein. In someexamples, management circuit 1602 is implemented by processor 108,controller 112, control circuit 204, and/or signal processing circuit208.

In some examples, management circuit 1602 may provide a command toprecision timing circuit 804 to adjust the measurement time window(e.g., by adjusting a temporal position of the measurement time windowand/or by adjusting a duration of the measurement time window). Thiscommand may be provided in response to user input, automatically inresponse to an event occurring within optical measurement system 100, aspart of a feedback loop in which the measurement time window is sweptacross the light pulse time period, and/or for any other reason.

Precision timing circuit 804 may be configured to adjust, based on atleast one signal generated within PLL circuit 802, a temporal positionof the measurement time window within the light pulse time period. Thismay be performed in any suitable manner. For example, as described inconnection with FIGS. 8-9 , VCO 806 may be configured to output aplurality of fine phase signals each having a different phase andfeedback divider 808 (e.g., LFSR 902) may be configured to have aplurality of feedback divider states during a PLL feedback period of thePLL circuit 802. Precision timing circuit 804 may be configured toadjust the temporal position of the measurement time window based on oneor more of the fine phase signals and one or more signals representativeof the feedback divider states.

To illustrate, the measurement time window may be defined as a sweep ofa plurality of timestamp symbols included in the timestamp signal busgenerated by timestamp generation circuit 1002 that occur during a PLLfeedback period, as described herein. The timestamp sweep may beconfigured to start each time LSFR 902 wraps (or at any other suitablefixed time within a PLL feedback period). As described herein, theoutput signal of phase intersection block 906-1 may be selectively usedas the PLL feedback signal provided to phase detector 810 to adjust thePLL feedback divider phase. This accordingly adjusts when the timestampsweep occurs, thereby adjusting the temporal position of the measurementtime window. Hence, to adjust the temporal position of the measurementtime window, management circuit 1602 may adjust the phase of the outputsignal generated by intersection block 906-1 and direct multiplexer 910to select the output signal generated by phase intersection block 906-1to be used as the PLL feedback signal.

Phase intersection block 906-1 may generate the output signal with theprogrammable phase in any of the ways described herein. For example, asdescribed herein, quadrature clock block 904 may be configured toselect, from the plurality of fine phase signals, four fine phasesignals that are quadrature shifted from each other for use asquadrature clock signals. Phase intersection block 906-1 may receive theplurality of fine phase signals, receive the quadrature clock signals,receive a programmable target state signal identifying a target feedbackdivider state included in the plurality of feedback divider states, andreceive a programmable target fine phase signal identifying a targetfine phase signal included in the plurality of fine phase signals andthat, in combination with the target feedback divider state, sets adesired phase of a pulse in the output signal. Phase intersection block906-1 may also generate a combination match signal when a currentfeedback divider state matches the target feedback state, use thequadrature clock signals to generate four registered match signalsrepresentative of the combination match signal, the four registeredmatch signals quadrature shifted from each other, select a particularmatch signal from the four registered match signal that is aligned witha pulse included in the target fine phase signal, and input the selectedmatch signal and the target fine phase signal into an AND gate to outputthe pulse of the output signal at a temporal position that correspondsto the desired phase.

Management circuit 1602 may be further configured to ensure that ameasurement time window is consistently placed with respect to eachlight pulse 1502 that is generated over the course of time in which aTPSF is generated. To do this, management circuit 1602 may be configuredto direct precision timing circuit 804 to sweep the measurement timewindow across the light pulse time period (e.g., some or all of thelight pulse time period) while the sequence of light pulses are beinggenerated. This sweeping may result in a TPSF being generated based ontimestamp symbols recorded by one or more TDCs while the measurementtime window is being swept. Management circuit 1602 may be furtherconfigured to determine a property of the TPSF and identify, based onthe property of the TPSF, a temporal location within the light pulsetime period (e.g., a temporal location that corresponds to thedetermined property of the TPSF). Management circuit 1602 may thendirect precision timing circuit 804 to adjust the temporal position ofthe measurement time window to align a particular time bin within themeasurement time window with the temporal location within the lightpulse time period. This process may be periodically repeated to ensurethat the measurement time window is consistently placed over time.

The TPSF property used in this process may be any suitable property ormetric as may serve a particular implementation. For example, theproperty may be a peak value of the TPSF, a full width at half maximummetric associated with the TPSF, a center of mass associated with theTPSF, a fitting metric associated with the TPSF, and/or across-correlation metric associated with the TPSF.

FIGS. 17A-17B illustrate an example in which the determined property isa peak value of the TPSF. In FIGS. 17A-17B, light pulses 1702 aredirected to a target, which scatters the light pulses 1702 beforephotons in the light pulses 1702 are detected by one or morephotodetectors. While a single pulse is shown in FIGS. 17A-17B, it willbe recognized that this single pulse represents multiple light pulses1702 that are sequentially applied and that each have a light pulse timeperiod 1704.

FIGS. 17A-17B also show an exemplary TPSF 1706 generated based ontimestamps recorded by one or more TDCs as the light pulses 1702 areapplied over time. TPSF 1706 is illustrated in dashed lines to connotethat TPSF 1706 is measured by aggregating a total number of photonsdetected in each time bin that follows an occurrence of each light pulse1702.

In some examples, after a chip on which the photodetectors and TDCs arelocated starts up, a measurement time window 1708 is positioned at arandom position due to the random phase that PLL locks to. As shown inFIG. 17A, this means that a reference time t within the measurement timewindow 1708 may be positioned at a random time bin with respect to theTPSF 1706. The reference time t may be any fixed temporal positionwithin measurement time window 1708.

Accordingly, management circuit 1602 may direct precision timing circuit804 to sweep the measurement time window 1708 across the light pulsetime period 1704 to identify a peak value of TPSF 1706. This sweep maybe performed by adjusting the temporal position of measurement timewindow 1708 across the light pulse time period 1704 over a sequence ofPLL feedback periods. The sweep may be performed with any suitableamount of time granularity. As the measurement time window 1708 is sweptacross the light pulse time period 1704, management circuit 1602analyzes the TPSF 1706 to identify a peak value of TPSF 1706. This maybe performed in any suitable manner.

Management circuit 1602 may determine a temporal location within lightpulse time period 1704 that corresponds to the identified peak value.This may be performed in any suitable manner. Once this temporallocation has been determined, as shown in FIG. 17B, management circuit1602 may direct precision timing circuit 804 to move measurement timewindow 1708 so that the reference time t is aligned with the peak valueof TPSF 1706. In this manner, the management circuit 1602 may ensurethat the measurement time window 1708 is consistently aligned duringTPSF generation.

The measurement time window may be adjusted at any suitable time duringoperation of optical measurement system 100. For example, managementcircuit 1602 may receive a command to perform a calibration of one ormore TDCs (e.g., at system startup and/or at any other time), and, inresponse, perform the measurement time window adjustment operationsdescribed herein. This calibration may be performed periodically overtime to ensure that the measurement time window is correctly positioned.

As mentioned, optical measurement system 100 may be at least partiallywearable by a user. For example, optical measurement system 100 may beimplemented by a wearable device configured to be worn by a user (e.g.,a head-mountable component configured to be worn on a head of the user).The wearable device may include one or more photodetectors and/or any ofthe other components described herein. In some examples, one or morecomponents (e.g., processor 108, controller 112, etc.) may not beincluded in the wearable device and/or included in a separate wearabledevice than the wearable device in which the one or more photodetectorsare included. In these examples, one or more communication interfaces(e.g., cables, wireless interfaces, etc.) may be used to facilitatecommunication between the various components.

FIGS. 18-23 illustrate embodiments of a wearable device 1800 thatincludes elements of the optical detection systems described herein. Inparticular, the wearable devices 1800 include a plurality of modules1802, similar to the modules shown in FIG. 6 as described herein. Forexample, each module 1802 includes a source 604 and a plurality ofdetectors 606 (e.g., detectors 606-1 through 606-6). Source 604 may beimplemented by one or more light sources similar to light source 110.Each detector 606 may implement or be similar to detector 104 and mayinclude a plurality of photodetectors. The wearable devices 1800 mayeach also include a controller (e.g., controller 112) and a processor(e.g., processor 108) and/or be communicatively connected to acontroller and processor. In general, wearable device 1800 may beimplemented by any suitable headgear and/or clothing article configuredto be worn by a user. The headgear and/or clothing article may includebatteries, cables, and/or other peripherals for the components of theoptical measurement systems described herein.

FIG. 18 illustrates an embodiment of a wearable device 1800 in the formof a helmet with a handle 1804. A cable 1806 extends from the wearabledevice 1800 for attachment to a battery or hub (with components such asa processor or the like). FIG. 19 illustrates another embodiment of awearable device 1800 in the form of a helmet showing a back view. FIG.20 illustrates a third embodiment of a wearable device 1800 in the formof a helmet with the cable 1806 leading to a wearable garment 1808 (suchas a vest or partial vest) that can include a battery or a hub.Alternatively or additionally, the wearable device 1800 can include acrest 1810 or other protrusion for placement of the hub or battery.

FIG. 21 illustrates another embodiment of a wearable device 1800 in theform of a cap with a wearable garment 1808 in the form of a scarf thatmay contain or conceal a cable, battery, and/or hub. FIG. 22 illustratesadditional embodiments of a wearable device 1800 in the form of a helmetwith a one-piece scarf 1808 or two-piece scarf 1808-1. FIG. 23illustrates an embodiment of a wearable device 1800 that includes a hood1810 and a beanie 1812 which contains the modules 1802, as well as awearable garment 1808 that may contain a battery or hub.

In some examples, a non-transitory computer-readable medium storingcomputer-readable instructions may be provided in accordance with theprinciples described herein. The instructions, when executed by aprocessor of a computing device, may direct the processor and/orcomputing device to perform one or more operations, including one ormore of the operations described herein. Such instructions may be storedand/or transmitted using any of a variety of known computer-readablemedia.

A non-transitory computer-readable medium as referred to herein mayinclude any non-transitory storage medium that participates in providingdata (e.g., instructions) that may be read and/or executed by acomputing device (e.g., by a processor of a computing device). Forexample, a non-transitory computer-readable medium may include, but isnot limited to, any combination of non-volatile storage media and/orvolatile storage media. Exemplary non-volatile storage media include,but are not limited to, read-only memory, flash memory, a solid-statedrive, a magnetic storage device (e.g. a hard disk, a floppy disk,magnetic tape, etc.), ferroelectric random-access memory (“RAM”), and anoptical disc (e.g., a compact disc, a digital video disc, a Blu-raydisc, etc.). Exemplary volatile storage media include, but are notlimited to, RAM (e.g., dynamic RAM).

FIG. 24 illustrates an exemplary computing device 2400 that may bespecifically configured to perform one or more of the processesdescribed herein. Any of the systems, units, computing devices, and/orother components described herein may be implemented by computing device2400.

As shown in FIG. 24 , computing device 2400 may include a communicationinterface 2402, a processor 2404, a storage device 2406, and aninput/output (“I/O”) module 2408 communicatively connected one toanother via a communication infrastructure 2410. While an exemplarycomputing device 2400 is shown in FIG. 24 , the components illustratedin FIG. 24 are not intended to be limiting. Additional or alternativecomponents may be used in other embodiments. Components of computingdevice 2400 shown in FIG. 24 will now be described in additional detail.

Communication interface 2402 may be configured to communicate with oneor more computing devices. Examples of communication interface 2402include, without limitation, a wired network interface (such as anetwork interface card), a wireless network interface (such as awireless network interface card), a modem, an audio/video connection,and any other suitable interface.

Processor 2404 generally represents any type or form of processing unitcapable of processing data and/or interpreting, executing, and/ordirecting execution of one or more of the instructions, processes,and/or operations described herein. Processor 2404 may performoperations by executing computer-executable instructions 2412 (e.g., anapplication, software, code, and/or other executable data instance)stored in storage device 2406.

Storage device 2406 may include one or more data storage media, devices,or configurations and may employ any type, form, and combination of datastorage media and/or device. For example, storage device 2406 mayinclude, but is not limited to, any combination of the non-volatilemedia and/or volatile media described herein. Electronic data, includingdata described herein, may be temporarily and/or permanently stored instorage device 2406. For example, data representative ofcomputer-executable instructions 2412 configured to direct processor2404 to perform any of the operations described herein may be storedwithin storage device 2406. In some examples, data may be arranged inone or more databases residing within storage device 2406.

I/O module 2408 may include one or more I/O modules configured toreceive user input and provide user output. I/O module 2408 may includeany hardware, firmware, software, or combination thereof supportive ofinput and output capabilities. For example, I/O module 2408 may includehardware and/or software for capturing user input, including, but notlimited to, a keyboard or keypad, a touchscreen component (e.g.,touchscreen display), a receiver (e.g., an RF or infrared receiver),motion sensors, and/or one or more input buttons.

I/O module 2408 may include one or more devices for presenting output toa user, including, but not limited to, a graphics engine, a display(e.g., a display screen), one or more output drivers (e.g., displaydrivers), one or more audio speakers, and one or more audio drivers. Incertain embodiments, I/O module 2408 is configured to provide graphicaldata to a display for presentation to a user. The graphical data may berepresentative of one or more graphical user interfaces and/or any othergraphical content as may serve a particular implementation.

FIG. 25 illustrates an exemplary method 2500 that may be performed byoptical measurement system 100 and/or any implementation thereof. WhileFIG. 25 illustrates exemplary operations according to one embodiment,other embodiments may omit, add to, reorder, and/or modify any of theoperations shown in FIG. 25 . Each of the operations shown in FIG. 25may be performed in any of the ways described herein.

In operation 2502, a photodetector generates a photodetector outputpulse when the photodetector detects a photon from a light pulse havinga light pulse time period.

In operation 2504, a TDC monitors for an occurrence of the photodetectoroutput pulse during a measurement time window that is within and shorterin duration than the light pulse time period.

In operation 2506, a precision timing circuit adjusts, based on at leastone signal generated within a PLL circuit, a temporal position of themeasurement time window within the light pulse time period.

FIG. 26 illustrates an exemplary method 2600 that may be performed bymeasurement time window management circuit 1602 and/or anyimplementation thereof. While FIG. 26 illustrates exemplary operationsaccording to one embodiment, other embodiments may omit, add to,reorder, and/or modify any of the operations shown in FIG. 26 . Each ofthe operations shown in FIG. 26 may be performed in any of the waysdescribed herein.

In operation 2602, a measurement time window management circuit directsa precision timing circuit to sweep a measurement time window across alight pulse time period associated with a sequence of light pulsesgenerated by a light source, the sweeping resulting in a TPSF beinggenerated based on timestamp symbols recorded by a TDC when the TDCdetects an occurrence of the light pulses while the measurement timewindow is being swept.

In operation 2604, the measurement time window management circuitdetermines a property of the TPSF.

In operation 2606, the measurement time window management circuitidentifies, based on the property of the TPSF, a temporal locationwithin the light pulse time period.

In operation 2608, the measurement time window management circuitdirects the precision timing circuit to adjust a temporal position ofthe measurement time window to align a particular time bin within themeasurement time window with the temporal location within the lightpulse time period.

An exemplary system described herein includes 1) a photodetectorconfigured to generate a photodetector output pulse when thephotodetector detects a photon from a light pulse having a light pulsetime period, 2) a TDC configured to monitor for the occurrence of thephotodetector output pulse during a measurement time window that iswithin and shorter in duration than the light pulse time period, 3) aPLL circuit for the TDC, and 4) a precision timing circuit connected tothe PLL circuit and configured to adjust, based on at least one signalgenerated within the PLL circuit, a temporal position of the measurementtime window within the light pulse time period.

An exemplary apparatus described herein includes a memory storinginstructions; and a processor communicatively coupled to the memory andconfigured to execute the instructions to: direct a precision timingcircuit to sweep a measurement time window across a light pulse timeperiod associated with a sequence of light pulses generated by a lightsource, the sweeping resulting in a TPSF being generated based ontimestamp symbols recorded by a TDC when the TDC detects an occurrenceof the light pulses while the measurement time window is being swept,determine a property of the TPSF, identify, based on the property of theTPSF, a temporal location within the light pulse time period, and directthe precision timing circuit to adjust a temporal position of themeasurement time window to align a particular time bin within themeasurement time window with the temporal location within the lightpulse time period.

An exemplary wearable system for use by a user includes 1) ahead-mountable component configured to be attached to a head of theuser, the head-mountable component comprising a photodetector configuredto generate a photodetector output pulse when the photodetector detectsa photon from a light pulse having a light pulse time period, 2) a TDCconfigured to monitor for the occurrence of the photodetector outputpulse during a measurement time window that is within and shorter induration than the light pulse time period, 3) a PLL circuit for the TDC,and 4) a precision timing circuit connected to the PLL circuit andconfigured to adjust, based on at least one signal generated within thePLL circuit, a temporal position of the measurement time window withinthe light pulse time period.

In the preceding description, various exemplary embodiments have beendescribed with reference to the accompanying drawings. It will, however,be evident that various modifications and changes may be made thereto,and additional embodiments may be implemented, without departing fromthe scope of the invention as set forth in the claims that follow. Forexample, certain features of one embodiment described herein may becombined with or substituted for features of another embodimentdescribed herein. The description and drawings are accordingly to beregarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. A system comprising: a time-to-digital converter(TDC) configured to monitor for an occurrence of a photodetector outputpulse during a measurement time window that is within and shorter induration than a light pulse time period, the photodetector output pulsegenerated by a photodetector when the photodetector detects a photonfrom a light pulse having a light pulse time period; a phase lock loop(PLL) circuit for the TDC and having a PLL feedback period defined by areference clock, the PLL circuit configured to: output a plurality offine phase signals each having a different phase; and output one or moresignals representative of a plurality of feedback divider states duringthe PLL feedback period; and a precision timing circuit connected to thePLL circuit and configured to: adjust, based on at least one of one ormore of the fine phase signals or the one or more signals representativeof the feedback divider states, a temporal position of the measurementtime window within the light pulse time period, and set, based on acombination of one of the fine phase signals and one of the feedbackdivider states, a temporal position of a timing pulse within the PLLfeedback period.
 2. The system of claim 1, wherein: the light pulse isincluded in a sequence of light pulses generated by a light source andeach having the light pulse time period; the system further includes ameasurement time window management circuit configured to direct theprecision timing circuit to sweep the measurement time window across thelight pulse time period while the sequence of light pulses are beinggenerated, the sweeping resulting in a temporal point spread function(TPSF) being generated based on timestamp symbols recorded by the TDCwhile the measurement time window is being swept, determine a propertyof the TPSF, identify, based on the property of the TPSF, a temporallocation within the light pulse time period, and direct the precisiontiming circuit to adjust the temporal position of the measurement timewindow to align a particular time bin within the measurement time windowwith the temporal location within the light pulse time period.
 3. Thesystem of claim 2, wherein the measurement time window managementcircuit is configured to periodically repeat the directing of theprecision timing circuit to sweep the measurement time window across thelight pulse time period, the determining of the property of the TPSF,the identifying of the temporal location within the light pulse timeperiod, and the directing of the precision timing circuit to adjust thetemporal position of the measurement time window.
 4. The system of claim2, wherein the measurement time window management circuit is configuredto: receive a command to perform a calibration of the TDC; and perform,in response to the command to perform the calibration of the TDC, thedirecting of the precision timing circuit to sweep the measurement timewindow across the light pulse time period, the determining of theproperty of the TPSF, the identifying of the temporal location withinthe light pulse time period, and the directing of the precision timingcircuit to adjust the temporal position of the measurement time window.5. The system of claim 2, wherein: the property of the TPSF comprises apeak value of the TPSF; and the temporal location within the light pulsetime period corresponds to a temporal position of the peak value.
 6. Thesystem of claim 2, wherein the property of the TPSF comprises one ormore of a full width at half maximum metric associated with the TPSF, acenter of mass associated with the TPSF, a fitting metric associatedwith the TPSF, or a cross-correlation metric associated with the TPSF.7. The system of claim 1, wherein: the PLL circuit is further configuredto generate a load signal; and the precision timing circuit comprises: aphase intersection block configured to generate, based on the fine phasesignals and the feedback divider states, an output signal with aprogrammable phase, and circuitry configured to selectively provideeither the load signal or the output signal to a phase detector includedin the PLL circuit; wherein the precision timing circuit is configuredto adjust the temporal position of the measurement time window byproviding the output signal to the phase detector.
 8. The system ofclaim 7, wherein the precision timing circuit further comprises: aquadrature clock block configured to select, from the plurality of finephase signals, four fine phase signals that are quadrature shifted fromeach other for use as quadrature clock signals; and the phaseintersection block is configured to: receive the plurality of fine phasesignals; receive the quadrature clock signals; receive a programmabletarget state signal identifying a target feedback divider state includedin the plurality of feedback divider states; receive a programmabletarget fine phase signal identifying a target fine phase signal includedin the plurality of fine phase signals and that, in combination with thetarget feedback divider state, sets a desired phase of a pulse in theoutput signal; generate a combination match signal when a currentfeedback divider state matches the target feedback state; use thequadrature clock signals to generate four registered match signalsrepresentative of the combination match signal, the four registeredmatch signals quadrature shifted from each other; select a particularmatch signal from the four registered match signal that is aligned witha pulse included in the target fine phase signal; and input the selectedmatch signal and the target fine phase signal into an AND gate to outputthe pulse of the output signal at a temporal position that correspondsto the desired phase.
 9. The system of claim 1, further comprising atimestamp generation circuit configured to: generate, based on a subsetof the fine phase signals that define a plurality of fine states for theplurality of fine phase signals, a timestamp signal bus representativeof a plurality of timestamp symbols that define the measurement timewindow; and transmit the timestamp signal bus to the TDC.
 10. The systemof claim 1, wherein: the TDC is included in a plurality of TDCs; the PLLcircuit is for all of the plurality of TDCs; and the measurement timewindow is for all of the plurality of TDCs.
 11. The system of claim 1,wherein the photodetector comprises a single photon avalanche diode(SPAD).
 12. The system of claim 1, wherein the photodetector is includedin a wearable device configured to be worn by a user.
 13. The system ofclaim 12, wherein the wearable device includes a head-mountablecomponent configured to be worn on a head of the user.
 14. A methodcomprising: generating a photodetector output pulse when a photodetectordetects a photon from a light pulse having a light pulse time period;monitoring for an occurrence of the photodetector output pulse during ameasurement time window that is within and shorter in duration than thelight pulse time period; and adjusting, based on at least one signalgenerated within a phase lock loop (PLL) circuit, a temporal position ofthe measurement time window within the light pulse time period.
 15. Themethod of claim 14, wherein: the light pulse is included in a sequenceof light pulses generated by a light source and each having the lightpulse time period; and the method further comprises: sweeping themeasurement time window across the light pulse time period while thesequence of light pulses are being generated, the sweeping resulting ina temporal point spread function (TPSF) being generated based ontimestamp symbols recorded while the measurement time window is beingswept, determining a property of the TPSF, identifying, based on theproperty of the TPSF, a temporal location within the light pulse timeperiod, and adjusting the temporal position of the measurement timewindow to align a particular time bin within the measurement time windowwith the temporal location within the light pulse time period.
 16. Themethod of claim 14, wherein the photodetector is included in a wearabledevice configured to be worn by a user.
 17. The method of claim 14,wherein the wearable device includes a head-mountable componentconfigured to be worn on a head of the user.
 18. A method comprising:directing, by a measurement time window management circuit, a precisiontiming circuit to sweep a measurement time window across a light pulsetime period associated with a sequence of light pulses generated by alight source, the sweeping resulting in a temporal point spread function(TPSF) being generated based on timestamp symbols recorded by atime-to-digital converter (TDC) when the TDC detects an occurrence ofthe light pulses while the measurement time window is being swept;determining, by the measurement time window management circuit, aproperty of the TPSF; identifying, by the measurement time windowmanagement circuit based on the property of the TPSF, a temporallocation within the light pulse time period; and directing, by themeasurement time window management circuit, the precision timing circuitto adjust a temporal position of the measurement time window to align aparticular time bin within the measurement time window with the temporallocation within the light pulse time period.
 19. The method of claim 18,further comprising periodically repeating the directing of the precisiontiming circuit to sweep the measurement time window across the lightpulse time period, the determining of the property of the TPSF, theidentifying of the temporal location within the light pulse time period,and the directing of the precision timing circuit to adjust the temporalposition of the measurement time window.
 20. The method of claim 19,further comprising: receiving, by the measurement time window managementcircuit, a command to perform a calibration of the TDC; and performing,by the measurement time window management circuit in response to thecommand to perform the calibration of the TDC, the directing of theprecision timing circuit to sweep the measurement time window across thelight pulse time period, the determining of the property of the TPSF,the identifying of the temporal location within the light pulse timeperiod, and the directing of the precision timing circuit to adjust thetemporal position of the measurement time window.